In optical communication systems and other optical application systems, optical devices, such as optical isolators, are indispensable components. For example, an optical isolator is utilized to prevent reflected light, which is reflected and back-traveling, from entering or being incident to optical elements. It is impossible to allow optical elements, such as high-speed optical amplifiers and high-speed laser diodes, to operate normally, without any optical isolators.
Integration of optical elements into optical integrated circuits (OICs) is necessary to lower production costs and to enhance the performance of high-speed optical data processing for high-speed optical networks. In further integration of the optical integrated circuits, reflected light that is reflected off from optical components and back-traveling causes unstable operation of a device in interest, resulting in a conspicuous problem. Optical isolators have the property of allowing light to be transmitted therethrough only in one direction, and have the function of blocking light reflected and coming back. Thus, it is important to integrate optical isolators into OICs. However, such the optical isolators are one of the optical components that are not integrated into OICs. At present, since no OICs having optical isolators integrated therein exist, OICs having optical isolators integrated therein will make a big market in the future.
Optical isolators require magneto-optical (MO) materials. Existing commercially-available optical isolators use transparent magnetic garnets. It is difficult to grow the magnetic garnets on substrates, such as Si, GaAs, and InP, which are used as substrates for OICs; however, some methods are reported. There are reports that optical isolators are fabricated on the substrates, by a method in which a magnetic garnet is bonded onto a semiconductor substrate, or by a method in which a magnetic garnet is sputtered onto a Si substrate. In addition, the inventors of the present invention have fabricated optical isolators on GaAs substrates using diluted magnetic semiconductors, such as CdMnTe and CdMnHgTe (see M. C. Debnath, V. Zayets, and K. Ando, “Thermal annealing of magneto-optical (Cd,Mn)Te waveguide for optical isolator with wider operation wavelength range”, Appl. Phys. Lett., 87, 091112 (2005), and JP-A-2005-315993 (“JP-A” means unexamined published Japanese patent application)). However, there is no report of any example in which other optical components are integrated on those semiconductor substrates having optical isolators integrated therein.
Magneto-optical properties of magnetic garnets and diluted magnetic semiconductors are affected by the quality of crystals of each of those. When the quality of magnetic garnets or diluted magnetic semiconductors is low, their magneto-optical constants become low to result in a high optical loss. Even if the crystal quality of those magneto-optical materials is maintained during the step of integrating an optical isolator into an OIC, the crystal quality of those magneto-optical materials becomes worse upon the step of integrating other optical components thereinto. This is the reason why any optical isolators, which are fabricated using magnetic garnets or diluted magnetic semiconductors, are not integrated into commercially-available OICs.
Ferromagnetic metals are expected to be favorable as magneto-optical materials for optical isolators. This is because ferromagnetic metals have high magneto-optical constants. Furthermore, the ferromagnetic metal fabrication technique, which is an important matter when fabricating OICs having optical isolators integrated thereinto, is compatible with the OIC fabrication technique. That is, since sputtering and lift-off deposition, which are generally used for OICs, can be applied to ferromagnetic metals, the quality of the ferromagnetic metals does become worse in the OIC fabrication process.
However, use of any of ferromagnetic metals has a drawback. The drawback is that ferromagnetic metals significantly absorb light. There is a method for solving this problem. The method is to compensate or off set the loss caused by the optical absorption by a ferromagnetic metal, by an optical gain of a semiconductor optical amplifier. The inventors of the present invention already proposed this method (W. Zaets and K. Ando, “Optical waveguide isolator based on non-reciprocal loss/gain of amplifier covered by ferromagnetic layer”, IEEE Photonics Technology Letters, vol. 11, pp. 1012-1014, August 1999), and we already realized the method (see V. Zayets and K. Ando, “Isolation Effect in Ferromagnetic-Metal/Semiconductor Hybrid Optical Waveguide”, Applied Physics Letter, vol. 86, pp. 261105, 2005.06). This type of optical isolator exhibits an excellent isolation function, and exhibits a low optical insertion loss. However, since its operation requires a large electric current of about 100 mA, such a large electric current is not acceptable for OICs. Due to this, it is impossible to have this type of optical isolator be integrated into OICs.
Further, the inventors of the present invention have studied on (Al,Ga)As optical waveguides having Fe embedded therein, which is a ferromagnetic metal (see V. Zayets, H. Saito, S. Yuasa, and K. Ando, “Magnetization-dependent loss in (Al,Ga)As Optical Waveguide with an Embedded Fe micromagnet”, Optics Letters, Vol. 35, pp. 931-933, 2010).
With respect to optical isolators, searching related to this application has been carried out. As a result, patent documents such as those below are found. WO 2009/067540 A1 discloses an optical device having a groove structure of a metal (Au, Cu, Ag), and using surface plasmons. Further, JP-A-2007-213004 discloses a device having a fine-particle arrangement layer including metal magnetic fine-particles. In the device, magnetization is generated by applying an external magnetic field to the metal magnetic fine-particles, and linearly polarized light is made incident in the device, so that a magneto-optical effect is caused by the interaction between the incident light to the metal magnetic fine-particles and the surface plasmon oscillation of the metal.
As described above, although there is a need to integrate efficient optical isolators or the like into optical integrated circuits, conventionally it was difficult to integrate optical devices, represented by optical isolators, into optical integrated circuits. To integrate optical isolators or the like into OICs, it is desirable to use, as a substrate, Si, InP, or GaAs, each of which is a semiconductor substrate. Furthermore, OICs having optical isolators integrated therein using such a substrate, needs to have properties of a high optical isolation and a low insertion loss.
In recent years, optical devices using plasmons have been proposed. The optical devices are also called plasmonic devices, and are devices which are ones of applications of plasmons excited, by allowing light or an electron beam to enter a metal. Optical waveguides using plasmons are called plasmonic waveguides or plasmon waveguides.
FIG. 12 shows the intensity of plasmons propagating along an interface between a metal and a dielectric. FIG. 12 is a diagram schematically showing a state in which the plasmons propagate along the interface between the dielectric and the metal. A wave-like arrow schematically represents the plasmons, and a transversely convex solid line represents the distribution of the plasmons.
Since light is tightly confined in the vicinity of the interface between the metal and the dielectric, the plasmons are used in many cases, for example, of integrated circuits where optical elements are integrated densely, or when light needs to be focused on a very small area, e.g. the cases of magneto-optical recording or medical field applications.
In the case of plasmons, since the optical intensity is partially distributed inside of the metal and inside of the dielectric, the plasmons always experience an optical loss. However, the optical loss of the plasmons is quite lower than the optical loss for light propagating through a bulk metal.
FIG. 13 is a graph showing propagation distance where plasmons attenuate to 1/e (vertical axis) as a function of a wavelength (horizontal axis, 0.7 μm to about 1.6 μm), in the case of using a low-resistance metal, such as Au and Cu. Those shown in FIG. 13 correspond to the case where the metal is Au and the dielectric is the air, and the case where the metal is Cu and the dielectric is the air, respectively in FIG. 12. In the cases of these metals, the propagation distance where plasmons attenuate to 1/e is long, about 200 to 500 μm. Due to this, these metals are used for circuits using plasmons. However, Au and Cu are not ferromagnetic metals. Fabrication of an optical isolator requires a ferromagnetic metal.
The cases of ferromagnetic metals Fe, Ni, Co, or the like will be described. FIG. 14 is a graph showing propagation distance where plasmons attenuate to 1/e (vertical axis) as a function of a wavelength (horizontal axis, 0.7 μm to about 1.6 μm), in the case of using a ferromagnetic metal, Fe, Ni, and Co. The curves shown in FIG. 14 show, in the order from the top of those, the case where the ferromagnetic metal is Ni and the dielectric is the air (Ni/air), the case where the ferromagnetic metal is Co and the dielectric is the air (Co/air), the case where the ferromagnetic metal is Fe and the dielectric is the air (Fe/air), the case where the ferromagnetic metal is Co and the dielectric is MgO (Co/MgO), and the case where the ferromagnetic metal is Co and the dielectric is AlGaAs (Co/AlGaAs). Since these ferromagnetic metals are high in resistance, the optical loss of the plasmons is also high, and the propagation distance where the plasmons attenuate to 1/e is significantly shorter than that of the cases of plasmons using a metal Au or Cu. The propagation distance is 50 μm at the longest, in the case where the ferromagnetic metal is Ni and the dielectric is the air. However, the high optical loss in this case can be resolved by shortening a device length. Since the magneto-optical constants of the ferromagnetic metals are high, even if the propagation distance where plasmons attenuate to 1/e is short, the plasmons can achieve a high optical isolation and a low insertion loss.
The magneto-optical (MO) figure-of-merit (hereinafter, also referred to as FoM) represents a ratio of optical isolation to insertion loss, and can be represented by the following equation:
      F    ⁢                  ⁢    o    ⁢                  ⁢    M    =            isolation      loss        =                            loss          forward                -                  loss          backward                                      (                                    loss              forward                        +                          loss              backward                                )                /        2            
The FoM indicates the performance or ability of plasmons to provide optical isolation corresponding to a value of required low insertion loss. The magneto-optical figure-of-merit is defined by the above equation, and thus is expressed in unit of %, but may exceed 100%. For example, the FoM is represented as follows: 0.3=30%, 1=100%, and 2=200%.
FIG. 15 is a graph showing a magneto-optical figure-of-merit (vertical axis), with a wavelength on the horizontal axis, in the case of plasmons propagating along the interface of Co/AlGaAs, Fe/air, Co/air, and Ni/air, respectively. A magnetic field is applied to perpendicularly to a propagation direction of the plasmons. In these four examples, the magneto-optical figure-of-merit is about 2 to 8%. Though these values are considerably favorable, there is a problem that the values are not yet sufficient to fabricate an efficient plasmonic optical isolator.